FIG. 1 shows a circuit configuration of a power converter using post-regulator(Mag-Amp) according to the prior art. As shown in FIG. 1, a prior art power converter using post-regulator(Mag-Amp) generally includes a switch control circuit 101, a magnetic amplifier 102, output rectifier 103, an output filter 104, a resistive-type voltage divider 105, and a reference voltage generator 106. The magnetic amplifier 102 is used to control the duty of an AC voltage VAC induced at the secondary side of the transformer T1 by way of magnetic coupling, and the resulting amplified voltage is converted into a DC voltage by the output rectifier 103. The resulting DC voltage is then filtered and smoothed by the output filter 104 comprised of an inductor LA and a capacitor CA, and thereby provided as an output voltage Vo for a load (not shown). In addition, a fraction of the output voltage Vo (which is indicated by a reference numeral VS) is sensed and sent to the switch control circuit 101 through a resistive-type voltage divider 105 made up of resistors RA and RB. The switch control circuit 101 uses an internal comparator (not shown for simplicity) to compare the fractional output voltage VS with a reference voltage Vref provided by a reference voltage generator 106, and in response thereto generates a control signal. An internal driving circuit of the switch control circuit 101 which is normally accomplished with a transistor drives the Mag-Amp 102 to control the duty of Vac, rectified by output rectifier 103, allowing the output voltage Vo to be stabilized. On the other hand, a housekeeping circuit (not shown in the drawings) is able to transmit a remote ON/OFF signal to the power converter to supply output voltage Vo to a load, such as a motherboard of a PC or a hard disk drive.
Some power supplies are able to provide multiple output voltages, each of which has a predetermined voltage level, for example, 5V or 3.3V, and each output voltage is sufficient to enable the appropriate operations of one or more electronic appliances that can operate under such output voltage. As shown in FIG. 2a, a first output voltage Vout1 of a power converter is increased up to a stable voltage level such as 5V with a fixed rising slope within its start-up period (0-t1) At this time, the power converter is operating under a start-up mode. The start-up period (or set-up period) referred to herein indicates a time period calculated from the moment at which an input terminal of the power converter is connected with a power source to the moment at which the output voltage becomes stable. Similarly, a second output voltage Vout2 of a power converter is increased up to a stable voltage level such as 3.3V with a fixed rising slope within its start-up period (0-t2), and the rising slope of the first output voltage Vout1 and the rising slope of the second output voltage Vout2 are quite close to each other. When both of the two output voltages are stabilized, the power converter can output two output voltages of different voltage levels to different loads, and then the power converter is driven to operate under a normal mode.
However, the load collocated with a power converter typically behaves as a dynamic load element. Therefore, in practical operation of a power converter, the load current of a power converter is also dynamically varying with time instead of being kept at a constant level. As a result, it is possible that the instantaneous load current variation causes the output voltage rising slope to change during a start-up period of a power converter. As shown in FIG. 2(b), the first output voltage Vout1 outputs an excessive voltage instantaneously by the influence of a corresponding load condition, so that the rising slope of the first output voltage Vout1 increases significantly. However, the rising slope of the second output voltage Vout2 remains constant, which may lead to a situation that a difference between an instantaneous voltage value of the first output voltage Vout1 and an instantaneous voltage value of the second output voltage Vout2 at time t3 is larger than a difference between a stabilized voltage value of the first output voltage Vout1 and a stabilized voltage value of the second output voltage Vout2, for example, 5V−3.3V=1.7V. That may cause the system to conduct faulty operations. More seriously, the whole power system may shut down accordingly. Such a phenomenon in which an instantaneous voltage value of one of the output voltages is sharply increased during the start-up period thus causing an improper operation of the power converter is generally called “rising slope out of control”.
In view of the foregoing descriptions, what is needed is a rising slope control technique for a power converter to obviate the occurrence of an “out of control” problem encountered during a start-up period of the power converter and ensure the proper operation of the system.